When you first sign up you will receive some pre race advise. You will receive a plan that you can choose to follow that will allow you to train for your chosen distance. I do recommend you follow the plan so you are ready for your swim. 
I remember the start of the swim jumping into the pool along with everyone else, I can remember thinking oh my what have I signed myself up for? That feeling goes very quickly and you soon get into the rhythm. My strong advise is to ensure you select the right speed lane. I think I misjudged myself and put myself too slow. Maybe have a practice and see how you feel when swimming. That was the only down side at the beginning. After those who had completed their swim I had a lot more room as I choose the 5K swim. 
You get such a strong sense of achievement afterwards and I raised far more than I expected. People got really behind me. At the time I was going through some mental health issues including losing my Nan. It really helped me through these difficult times. 
I recommend you give it a try you'll have lots of fun!